Permanent magnet brakes – high power density and high dynamics

The typical operating principles of permanent magnet and spring-applied brakes and their typical friction pairs – steel on steel for permanent magnet brakes versus organic friction linings on steel for spring-applied brakes – result in defined key features and typical fields of application of the two brake types.

Permanent magnet brakes are ideal for servo motors used on handling equipment and robots, for example. Their compact dimensions and relatively low weight make them the perfect solution for these applications. Owing to the use of permanent magnets, their power density is twice the density of spring-applied brakes. In addition to their low weight and minimal abrasion, dynamic permanent magnet brakes offer additional benefits that make them the number one choice in robotics. The abrasion resistance of permanent magnet brakes is the result of their typical operating principle. The armature is entirely released by the spring.

In spring-applied brakes, wear occurs during starting because the speed increase requires an air buffer to be created between the friction lining and the friction surfaces. Additional wear may occur if the friction disc accelerates as a result of gravitational acceleration in vertical drive arrangements or as a result of centrifugal forces during rotation of the rotor blades of wind turbines, for example. However, this kind of wear usually affects only one friction lining. When used as a mere holding brake with emergency stop function, the behaviour of permanent magnet brakes is different compared to spring-applied brakes. Owing to its specific design, the permanent magnet brake has zero residual torque. Abrasion only occurs during emergency stops. During operation, the armature is completely released by the spring. By contrast, the spring-applied brake requires a starting torque, which produces a certain amount of wear at each start. As already mentioned, additional wear occurs as a result of acceleration forces. In many cases, this additional wear cannot be determined accurately because, in general, only one side of the friction disc is affected.

Permanent magnet and spring-applied brakes also differ in terms of their behaviour across a specific temperature range. Permanent magnet brakes have excellent temperature stability and provide a constantly high torque across the entire temperature range. The situation is different with spring-applied brakes. Their temperature stability is decisively determined by the composition of the organic friction lining. In a way, this can be compared to the different types of car tyres developed for different applications. Similarly to a Formula 1 tyre which cannot be used in winter, some organic friction linings of brakes are simply not suitable for certain applications. Friction linings with a high coefficient of friction are characterized by good adhesion. The torques that can be achieved are high, but the friction linings are subject to early wear. As far as the friction linings in spring-applied brakes are concerned, this means that linings with high coefficients of friction show faster torque reduction over the entire temperature range. In some cases the torque may drop to 50% at 120°C or -40°C. In general, it can be said that spring-applied brakes either reach excellent torques at the expense of their temperature stability or that their friction linings have excellent temperature stability at the expense of their coefficient of friction. However, it is worth noting that based on a given temperature range the torque of spring-applied brakes can be accurately rated for the customer-specified torque during the design process.